Perception of extracellular signals by cell surface receptors is of central importance to eukaryotic development and immunity. Cell surface Pattern Recognition Receptors (PRRs), play an essential role in the innate immune responses in animals and plants. PRRs share conserved signaling domains, such as leucine-rich repeats (LRRs), and function via kinases, which are either integral to the receptor (plants) or associated with the receptor (animals). Many of these receptors regulate transcription of target genes through phosphorylation events after recognition of pathogen associated molecular patterns (PAMPs).
In animals, recognition of PAMPs in extracellular compartments or at the cell surface is largely carried out by the toll like receptor (TLR) family that contains LRRs in the extracellular domain and a TIR intracellular domain. All TLRs activate a common signaling pathway to induce a core set of defense responses via kinases. TLRs associate with the interleukin-1 receptor-associated kinase (IRAK) family and with receptor interacting-protein (RIP) kinases via adaptor proteins. In plants, cell surface recognition of PAMPS is largely carried out by receptor kinases (RKs).
Kinases can be divided into “RD” kinases, which have an arginine (R) immediately preceding the conserved catalytic aspartate (D), and a smaller group of “non-RD” kinases, which typically carry a cysteine or glycine before the aspartate residue. The RD class of kinases includes nearly all receptor tyrosine kinases (RTKs) and most characterized plant RSTKs. The non-RD class includes members of human IRAKs and RIPs, Drosophila Pelle, and members of plant RKs belonging to the IRAK family.
Plant genome analyses have revealed the presence of a large family of these non-RD IRAK RKs, with more than 45 encoded in the Arabidopsis genome and more than 370 found in the rice genome. Members include Arabidopsis FLS2 that detects the peptide flg22, a component of flagellin; the elongation factor Tu receptor (EFR) that detects elf18, a peptide corresponding to the bacterial elongation factor Tu; the rice XA26 and Pi-d2 RKs; and the rice XA21 RK that mediates recognition of Xanthomonas oryzae pv. oryzae (Xoo).
Activation of XA21 results in a robust immune response. XA21 biogenesis takes place in the endoplasmic reticulum. After processing and transit to the plasma membrane, XA21 binds to XB24 (XA21 binding protein 24). XB24 associates with the XA21 juxtamembrane domain and catalyzes the autophosphorylation of serine and threonine residues on XA21, maintaining XA21 in an inactive state. Following pathogen recognition, the XA21 kinase disassociates from XB24 and is activated. XA21 also binds to XB25, a plant-specific ankyrin-repeat protein. XB25 associates with the XA21 transmembrane domain in vitro and is weakly transphosphorylated by XA21. The XA21-mediated signaling pathway is attenuated by XB15, a protein phosphatase 2C, which dephosphorylates XA21. XA21-mediated immunity requires the presence of a co-receptor called OsSERK2 (somatic embryogenesis receptor kinase 2). OsSERK2 interacts with the intracellular domain of XA21 in a kinase activity dependent manner. OsSERK2 undergoes bidirectional transphosphorylation with XA21 in vitro and forms a constitutive complex with XA21 in vivo.